Electron gun having tapered emitting cathode surface



J. H. o. IHARRIES A rifzo, 1965 ELECTRON GUN HAVING TAPERED EMITTING CATHODE SURFACE 3 Sheets-Sheet 1 Filed Nov. 24, 1959 lF e a liq/ m l6 April 20, 1965 J. H. o. HARRIES ELECTRON GUN HAVING TAPERED EMITTING CATHODE SURFACE 5 Sheets-Sheet 2 Filed NOV. 24, 1959 A ril 20, 1965 Y J. H. o. HARRIES ELECTRON GUN HAVING TAPERED EMITTING CATHODE SURFACE 5 Sheets-Sheet 3 Filed Nov. 24, 1959 United States Patent 3,179,844- ELECTRON GUN HAVING TAPERED EMlG CATHQDE SURFAE John Henry Owen Harries, Warwick, Bermuda, assignor to Harries Electronics Corporation Limited Filed Nov. 24, 195i Ser. No. 855,133 Claims priority, application Great Britain, Nov. 25, 1958, 37,954/58 7 Claims. (Cl. 3115-31) This invention relates to electron guns for electron discharge tubes of the kind which provides a visual display, for example tubes of the kind used for television, radar or cathode ray Oscilloscopes. The invention is particularly useful in discharge tubes in conjunction with optical projection systems although the advantages of the invention are not confined to such applications.

Display systems for television, radar, cathode ray oscilloscopes and the like have been proposed in which an electron image, or raster, is produced on a phosphor screen by scanning the latter with an electron beam in an electron discharge tube, and in which an optical image of the electron image, or raster, is'projected on to a viewing screen by an optical system. As an example, an optical system of the Schmidt kind, employing a concave mirror and a correcting lens, can be used.

We have found that the properties of display systems of this kind are such that the component parts are more compact and much less costly, and the reproduced image quality is generally improved, if the electron image, or raster, is very much smaller than those commonly used in the prior art; thus, for example, we have found it ver advantageous to use rasters which are twenty to thirty times smaller in linear dimensions than a projected optical image of size 14 inches by 17.3 inches. The optical components and other elements of the system are then small and the system as a whole is more compact than would be the case if the electron image, or raster, were comparable in size to the projected optical image. Moreover, good optical quality is more readily obtainable in small optical elements than in large ones. In yet other display systems, for example those used for oscillographs where the raster is photographed, a small faster is also desirable.

In the prior art, however, there has been a severe limitation 'to the reduction which is possible in the size of the electron image or raster. This limitation stemmed V from a fundamental inability of known electron guns used in display systems to produce electron beams of sufficiently small diameter at the focal spot on the phosphor screen with adequate beam current. As a consequence, it

was not possible (for a given number of scanning lines and definition of the electron image or raster) to reduce the size of the raster 'as far as is desirable.

According to the'invention, the electron gun includes a shieldingelectrode in which an aperture is formed, and a cathode having an emitting part which is tapered and of axially symmetric shape andwhich passes through the aperture and projects beyond the edges'of the aperture whereby an electron beam can be obtained in which the r paths of electrons leaving the'said emitting partof the cathode diverge'from the axis of the gun. The term axial symmetry is intended to mean symmetry about any plane containing the axis ofthe electron gun. The projecting portion of the cathode surface 'is preferably brought to a sharp axially symmetric point. The surface of the shielding electrode around the aperture may be plane and perpendicular to the axis of the electron gun, or can be of a conical form with the apex of the cone point-mg towards the focusingelectrodes When a display tube including such a gun is,connected in a video output circuit,

appropriate potentials are applied to the focusing elcctrodes in order to focus the electron emission from, the

ice

cathode to form a narrow electron beam which becomes a small focused spot on the phosphor screen at a distance from the cathode. This electron beam is deflected and scanned over the phosphor screen by electric or magnetic fields in the usual manner to produce an electron image or raster on the phosphor screen of the tube. The optical image of this electron image or raster may be viewed directly or may be projected on to the viewing screen by a suitable optical system. The shielding electrode may be used as a modulator electrode to control the beam current.

In prior cathode and modulator electrodes for use in display systems, the electrons emitted from the cathode have been sharply converged immediately in front of the cathode and modulator to a cross-over point. The electrons then diverge again, after which they are focussed by an electron lens on to the phosphor screen. This sharp convergence immediately in front of the cathode is primarily due to the fact that the cathode does not project through the aperture in the modulator, the potential of which is negative with respect to the cathode and is varied to modulate the electron beam current. It has been taught in the prior art that this sharp convergence immediately in front of the cathode tends to produce a desirably small spot size at the phosphor screen, but, on the contrary, applicant has found that these cathode and modulator systems do not succeed in this respect. Applicant believes that this is because the sharp convergence of the electrons produces severe electron optical aberrations which increase the spot size, and also because the electron trajectories from the cathode through the electron lens system are such that the focussed spot of electrons at the phosphor screen is an image of a virtual or effective object which is positioned no further from the electron lens than, for example, the neighbourhood of the cathode. This relatively short object distance results in a relatively high lens magnification and, therefore, also leads to a relatively large focussed spot.

Applicant has found that electron guns in accordance with this invention can produce a smaller spot size at the phosphor with a given electron lens than is possible with these prior cathode and modulator systems. He believes that this improvement is partly because, contrary to the teaching of the prior art, the arrangement of the cathode and the shielding electrodes in accordance with our invention is not such as sharply to converge the electrons immediately in front of the cathode shielding electrodes. It is believed that the improved and reduced spot size at the phosphor is obtained because aberrations are then substantiallyavoided and because the electron trajectories are such that the focussed spot of electrons atjthe phosphor screen is an image of a virtual or effective object 1 which is positioned much further from the electron lens 7 than is the case with the prior art; hence'the magnification of the electron lens system is reduced thereby producing a smaller focussed spot at thephosphor screen.

The necessary modulation or control of the electron beam current may be obtained by using the transconductance of the apertured electrode through which the cathode passes, that is to say, by varying the potential of the shielding electrode, which is now the modulator, with respect to the cathode. It might be thought that the control ofthe beam current by the modulator potential would be greatly reduced by the use of a cathode point which projects through the modulator aperture and extends beyond the modulator surface', as compared with the control obtained when the cathode does not so project through the aperture in the modulator and that as a result greater controllingpotentials would be required. However it is found that this reduction of control is not signifi: cant provided that the diameter'of the aperture with U respect to the protrusion of the cathode point is not too great.

The shielding electrode may be cup-shaped and the cathode may be centred within this electrode by placing a locating member around the cathode within the shielding cup and passing adjusting screws through apertures in the cup, so that rotation of the adjusting screws moves the locating member with respect to the axis of the cup. The electrodes of the electron gun maybe aligned and positioned by passing a mandrel through the apertures in the electrodes and placing spacing washers on the mandrel between successive electrodes. The mandrel is inserted from the end opposite the cathode, and the end of the mandrel which first enters the electrode assembly has a hollow cavity which surrounds the cathode when the mandrel is in position with its leading end in the aperture in the shielding electrode.

In order that the invention may be better understood, embodiments thereof will now be described with reference to the accompanying drawings, in which:

FIGURE 1 is a sectional view of an electron gun constructed in accordance with the present invention;

FIGURE 2 shows a modification of the gun of FIG- URE 1;

FIGURE 3 illustrates in section an electron discharge tube incorporating the gun of FIGURE 1;

FIGURES 4A, 4B and 4C illustrate a method of centring the cathode Within the modulator electrode;

FIGURE 5 illustrates a method of aligning the electrodes of the electron gun; and

FIGURE 6 is a detailed view of a part of FIGURE 5.

In FIGURE 1, the cathode sleeve 1 has a conical axially symmetric point 2 which projects through an aperture 3 in a cup-shaped electrode 4 (which can be used as a modulator if desired), the tip of the cathode extending beyond the surface of the shielding electrode by a distance h. The electrode 4 shields the cathode so that only a small portion of the latter is exposed to the positive field provided by subsequent anodes. The cathode face and the point 2, which should preferably be as sharp as possible are covered with an emissive coating of suitably small grain size to preserve the shape of the point. The cathode is supported inside the shielding cup 4 by a ceramic disc 5 which is clamped between a spacer 6 and a retaining washer 7 which is spot welded to the sides of the cup 4. A re-entrant wound helical heater 8 is positioned inside the cathode sleeve to heat the cathode.

A first anode electrode 9 containing an axial aperture 11 is spaced from the shielding electrode 4 by a distance b, and a second anode electrode containing an axial aperture 12 is spaced from the first anode electrode 9 by a distance c. The anode 10 also includes an axial beamlimiting aperture 13 through which the electrons travel to a phosphor screen 17. The electrodes, including the cathode, have axial symmetry around the axis 14 of the electron gun and are supported by metal spigots 15 which are embedded in four insulating glass rods 16. For clarity only two of these rods are shown in FIGURE 1.

With the arrangement described, electrons leaving the cathode point 2 tend to travel in diverging paths in the neighbourhood of the apertures 3 and 11. The resulting electron beam is limited in cross-sectional area by the limiting aperture 13 and travels along the axis towards the phosphor screen 17. An electron lens and electrondeflecting means (not shown) are positioned in the space between the second anode 10 and the phosphor screen 17. The electron beam is focussed by the electron lens to a small spot on-the phosphor screen, and the electrondetlecting means may be used to cause this spot to scan over the phosphor screen to produce a raster. It is important that the conical point 2 of the cathode, the apertures 3, 11 and 12, and the electrodes in general should be of substantially axially symmetric shape in order to minimise aberrations of focus and toipro'duce a small focussed spot at the phosphor screen.

In FIGURE 1 the thickness of the walls of the electrodes, the diameters of the apertures and size of the cathode point have been exaggerated for the sake of clarity. FIGURE 2 illustrates a modification of FIGURE 1 whereby the apertured end surface 4a of the shielding electrode 4 is of substantially conical form, the apex of the cone lying on the axis 14 of the system and pointing towards the phosphor screen. This configuration tends to increase the initial divergence of the electrons in the neighbourhood of the cathode.

FIGURE 3 shows in section an electron discharge tube having an electron gun 18 of the kind shown in FIGURE 1 and an optical system to project on to a viewing screen animage of a faster on the phosphor screen or ultor 17a within the tube. The tube includes a vacuum envelope 19, within which a concave spherical mirror 20 is arranged between the electron gun 18 and the phosphor screen 17a and is provided with a central aperture 21 through which the electron beam e from the gun 18 passes on its way to the phosphor screen 17a. The spherical mirror 20, which is axially arranged with respect to the electron gun and phosphor screen, faces the phosphor screen and reflects light from the latter back past the phosphor screen and through the transparent faceplate 22 of the envelope 19. The phosphor screen 17a and the locating pin 23 are respectively supported upon spider arms 24 and 25 which intercept little of the light. After emerging from the vacuum envelope, the light rays pass through the limiting aperture 26 and a corrector meniscus 27 and then travel to the viewing screen 28 on which they form a magnified image of the raster on the phosphor screen 17a. This electron discharge tube and optical system is of the same general kind as that described in co-pending patent application Serial No. 780,421, now Patent No. 2,960,615. As a result of the use of the electron gun described above the phosphor mount 17a may be made relatively small and the optical magnification may be made large (say 25), thus attaining the advantages previously mentioned.

As stated above, the shielding cup 4 may be used as a beam-modulator electrode. Typical potential values for a tube in which the shielding electrode is used asa modulator are as follows:

1st anode 200 volts. 2nd anode 5,000 volts. Ultor 30,000 volts.

It is important to the attainment of the smallest possible focussed spot that the point 2 of the cathode 1 be truly concentric with the circular aperture 3 in the shielding electrode 4. Therefore, after the ceramic disc 5 in FIGURE 1 upon which the cathode 1 is mounted has been clamped between the spacer '6 and retaining washer 7 inside the cup 4, the ceramic disc 5.is preferably adjusted transversely to the axis 14 whilst the modulator and cathode are examined under a microscope or other optical inspecting device. Preferably the surface of the retaining washer 6 which engages the ceramic disc 5 is serrated or roughened to improve its .grip on the ceramic. FIGURE 4A shows a shielding cup 4 which has four holes 49 through the sides so that the ceramic disc 5 is exposed as shown. After clamping the ceramic disc 5 and cathode 1 inside the shielding cupby means of a spacer washer 6 and retaining washer 7 (FIGURE 1), the assembly as a whole is placed inside a hollow jig 51 as shown in FIG- URES'4B and 4C. The cup 4 is retained inside the hollow jig by means of the screwed plug 52. The four screws 50 engage the exposed portions of the ceramic disc 5 through the holes 49 in the cup 4. By adjusting the screws 50 the cathode point 2 may be centred precisely within the aperture 3. This adjustment is performed whilst the cathode point 2 and aperture 3 are viewed by a microscope having a measuring graticule and having its optical axis in the direction of the arrow 53 in FIG- URE 4C.

The height h (FIGURE 1) by whichjthe cathode point 2 extends beyond the surface of the shielding cup 4 may be set by choosing the height of the spacerwasher 6.

The performance of the above described electron gun is further improved if a high degree of accuracy is attained in the alignment of the aperture in the shielding cup and the other apertures and electrodes between the cathode and the phosphor.

In order to obtain this accuracy by simple means, I provide that during assembly of the electrodes they are supported on a mandrel which passes through the various anode electrodes and also through the aperture in the shielding cup. This arrangement is illustrated in FIG- URE 5, in which the electrodes of the electron gun shown in FIGURE 1 are mounted upon a mandrel 29. The cup 4 is supported by the support 30. The distances b and 0 (FIGURE 1) are maintained by means of the spacer washers 31, 32. The mandrel 29 is mounted in a hearing 33 and the support 30 is mounted in a second bearing 34. The mandrel-29 is free to move in the direction of the arrow 35. The support 30 is prevented from moving in the direction of the arrow 35 by means of the collar 3611 which is fixed to the support 30 by means of the set screw 42.. The collar 36 can slide freely on the mandrel 29 and the collar 37 may be fixed by means of the set screw 38 in any desired position between the collar and the bearing 33. The spring 39 presses the electrodes and spacing washers 31, 32 together and upon the face of the modulator cup 4. The spring 40 presses the mandrel in the direction of the arrow 35 so that it pushes on the first anode 9 and spacer washer 31. The cavity 47 in the support 30 is arranged to accommodate the contact strip 48 attached to the cathode 1.

The mandrel 29 has an extension 43 which passes through the aperture 11 in the first anode 9 and through the aperture 3 in the cup electrode 4. A cavity 44 is provided in the extension 43 to accommodate the point 2 of the cathode.

The assembly as a whole may be rotated in the bearings 34, 33 around the axis 41 and the spigots are embedded in insulating glass support rods 16 (FIGURE 1) while these are heated to soften them in a manner well known U in the art. The mandrel is then withdrawn, the heater 8 inserted, and the assembly becomes that shown in FIG- URE 1.

For the sake of clarity the thickness of the walls of the and second anodes 9 and 10 are fed on the mandrel 29v with the spacer washers 31, 32. Then the modulatorcup 4 is positioned on the support'30 and the mandrel exten sion 43 slid into the aperture 3 in the modulator electrode 4. It will be understood that unless precautions are taken there will be apossibility of the projectingtip' of the cathode being damaged as the extension 43 slides into the aperture 3 in the cup 4. To prevent this from happening the support 30'is provided with an extension 45, the diameter of which is slightly less than the internal diameter of the cup 4. The diameter of the extension is such that the outside of the extension 45 is less than the space between the point of the cathode and the inside of the cavity 44 when the extension 43 is just entering the aperture 3 in by the extension 43015 the mandrel 29. At the same time,

the existence of the annular gap 46 when the extension 43 is in the aperture 3 prevents the extension 45 of the support 30 from determining the position of the cup 4 with respect to the other electrodes. As a result the axial alignment of the apertures in the electrodes and therefore that of the electrodes themselves is assured by the mandrel, and this alignment is not interfered with by the extension 45 which, nevertheless, enables assembly to be performed rapidly without damage to the point of the cathode.

FIGURE 6 shows the cathode point 2 and the cathode I and the cathode coating 2a in greater detail. The cathode may consist of nickel and the coating may consist of the usual BrSr oxides. The shielding electrode 4 and the first anode 9 are shown in FIGURE 6 with the extension 43 of the mandrel 29 passing through them but the spacer washer 31 of FIGURE 5 has been omitted for clarity. The cavity 44 accommodates the cathode point as described above. The thickness d of the wall of the shielding electrode 4 (FIGURE 6) is made sufficient to provide adequate bearing surface for the extension 43. The distance a is made sufiiciently great to avoid any danger of the extension 43 of the mandrel 29 touching the cathode surface. This may be compared with prior electron guns using cathodes which are plane and have no point. Aberrations, which result in an unduly large .focussed spot size are caused by these guns unless the distance from the cathode face to the inside of the modulator and the thickness of the modulator wall are both minimised. In consequence, these distances have to be too small for an extension of a mandrel to be inserted through the modulator aperture without damage to the cathode. Less direct methods of aligning this aperture with the other apertures and electrodes have to be used. Applicant has found that these methods are relatively ineffective and that the focussing spot size is correspondingly increased in size and varies seriously from gun to gun of this prior kind.

Referring to FIGURES 1 and 6 typical dimensions may be as follows:

The thickness d of the wall of the shielding electrode 4 and the thickness e of the wall of the first anode 9 may both be 0.010. The distance a may be 0.0075".

The distance b between the electrode 4 and the first anode 9 may be 0.040. The distance c in FIGURE 1 may be about 0.040". The cone dimensions (FIG- DRE 6) may be A=0.020" and B=0.022". The distance h by which the tip of the cathode is proud of the face of the shielding electrode 4 may be 0.00 to 0.004". It will be understood that these are typical dimensions, but that they may be varied to suit the conditions of focus and design.

I claim:

1. An' electron gun for an electron discharge display tube including a thermionic cathode, a hollow beam current control electrode shieldingsaid cathode and terminating in' a surface which is substantially normal to the axis of the electron gun and in which a central aperture is formed, said thermionic cathode having an emitting part which tapers to a point and is of axially symmetric shape and which passes through the aperture and projects beyond the edges of the aperture, whereby said shielding electrode provides sensitive control of said beam current and the paths of electrons leaving the said emitting part of the cathode diverge from the axis of the gun.

2. An electron gun for an electron discharge display tube including a thermionic cathode, a hollow beam curannular space 46 between the inside of the cup 4 and the and is of axiallysymmetric shape and which passes rent control electrode shielding said cathode, and at least the end of which is of substantially conical form with the apex of the cone on the axis of the electron gun and which has an axial aperture at the said apex, said thermionic cathode having an emitting part which tapers to a point through the aperture and projects beyond the edges of the aperture, whereby said shielding electrode provides sensitive control of said beam current and the paths of electrons leaving the said emit-ting part of the cathode diverge from the axis of the gun.

3. An electron discharge display tube comprising an electron gun including a thermionic cathode, a beam current control electrode shielding said cathode and in which an aperture is formed on the axis of the gun, said thermionic cathode having an emitting part which tapers to a point and is of axially symmetric shape and which passes through the aperture and projects beyond the edges of the aperture, said tube further comprising beamfocusing means and a fluorescent screen mounted on the axis of said gun, whereby there is obtained an initially divergent electron beam which is focused to a small cross-section at the fluorescent screen.

4. An electron gun for an electron discharge display tube including a thermionic cathode, a hollow beam current control electrode shielding said cathode, and having an end portion which is directed towards the axis of the electron gun and which has a central aperture on said axis, and said thermionic cathode having an emitting part of conical shape terminating in a point and passing through the aperture so as to project beyond the edges of the aperture.

5. An electron discharge display tube including an electron gun having a thermionic cathode and a beam current control electrode which shields said cathode and which has an end surface in which a central aperture is formed, said thermionic cathode having an emitting part of axially symmetric shape which tapers to a point and passes through the aperture to project beyond the edges thereof, whereby said shielding electrode provides sensitive control ofthe beam current and the paths of electrons leaving the said emitting part of the cathode diverge from the axis of the gun.

6. Visual display apparatus including an electron discharge display tube comprising an electron gun having a thermionic cathode and a beam current control electrode shielding said cathode and having an end surface in which a central aperture is formed, said thermionic cathode having an emitting part of axially symmetric shape which tapers to a point and passes through the aperture to project beyond said end surface, said display tube further comprising focusing means and a fluorescent screen, means for applying to said control electrode a biasing potential which is negative with respect to the potential of said cathode, means for applying modulating potentials to said control electrode and means for applying to said focusing means potentials such as to focus an electron beam from the cathode to a small spot at the fluorescent screen.

7. An electron gun according to claim 5 in which the tapered part of the cathode is conical in shape.

References Cited by the Examiner UNITED STATES PATENTS 2,173,165 9/39 Headrick. 2,223,040 11/40 Mabl 313-336 X 2,227,051 12/40 Wienecke 313-82 X 2,227,070 12/40 Boer 88-14 2,363,359 11/44 Ramo 313-821 2,520,190 8/50 Amdursky 313-92 2,625,734 l/53 Law 29-2513 2,632,130 3/53 Hall 313-821 2,640,950 6/53 Cook 313-85 X 2,767,457 10/56 Epstein 29-2513 2,886,729 5/59 May 313-821 2,938,134 5/60 Levin 313-82 2,947,897 8/60 Rodriguez et al. 313-82 3,013,171 12/61 Beck 313-82 3,065,374 11/62 Rockwell 313-82 X FOREIGN PATENTS 736,216 9/55 Great Britain. 420,067 11/34 Great Britain.

GEORGE N. WESTBY, Primary Examiner. RALPH G. NILSON, ARTHUR GAUSS, Examiners. 

1. AN ELECTRON GUN FOR AN ELECTRON DISCHARGE DISPLAY TUBE INCLUDING A THERMIONIC CATHODE, A HOLLOW BEAM CURRENT CONTROL ELECTRODE SHIELDING SAID CATHODE AND TERMINATING IN A SURFACE WHICH IS SUBSTANTIALLY NORMAL TO THE AXIS OF THE ELECTRON GUN AND IN WHICH A CENTRAL APERTURE IS FORMED, SAID THERMIONIC CATHODE HAVING AN EMITTING PART WHICH TAPERS TO A POINT AND IS OF AXIALLY SYMMETRIC SHAPE AND WHICH PASSES THROUGH THE APERTURE AND PROJECTS BEYOND THE EDGES OF THE APERTURE, WHEREBY SAID SHIELDING ELECTRODE PROVIDES SENSITIVE CONTROL OF SAID BEAM CURRENT AND THE PATHS OF ELECTRONS LEAVING THE SAID EMITTING PART OF THE CATHODE DIVERGE FROM THE AXIS OF THE GUN. 